1. Field of the Invention
The present invention relates to power switching processes and devices, and more particularly, to resonance-type power switching techniques and devices.
2. Description of the Related Art
Generally, a switching mode power supply (SMPS) is a DC-DC converter constructed with a string of switching elements and a switching controller for controlling the switching operation of the switching elements. Recent designs found in the art include U.S. Pat. No. 5,654,881 issued to Albrecht et al., entitled Extended Range DC-DC Power Converter Circuit, U.S. Pat. No. 5,559,684 issued to Ohms et al., entitled Switching Regulator, U.S. Pat. No. 5,515,258 issued to Viertler, entitled Drive Device For A Push-Pull Stage, U.S. Pat. No. 5,438,500 issued to Ohms, entitled Switching Regulator With A Push-Pull Resonance Converter, U.S. Pat. No. 5,045,712 issued to Baggenstoss, entitled Synchronized Switched Mode Power Supplies, U.S. Pat. No. 4,937,468 issued to Shekhawat et al., entitled Isolation Circuit For Pulse Waveforms, U.S. Pat. No. 5,159,541 issued to Jain, entitled Asymmetrical Pulse Width Modulated Resonant DC/DC Converter, U.S. Pat. No. 5,317,496 issued to Seiersen, entitled DC/DC-Converter With A Primary Circuit And At Least One Secondary Circuit Tuned As Individually Oscillatory Circuits, U.S. Pat. No. 4,901,215 issued to Martin-Lopez, entitled Isolated Switch Mode Power Supply Controller, U.S. Pat. No. 5,287,262 issued to Klein, entitled High Voltage Resonant Inverter For Capacitive Load, U.S. Pat. No. 5,572,418 issued to Kimura et al., entitled Quasi-Resonant PWM Inverter, and U.S. Pat. No. 5,546,294 issued to Schutten et al., entitled Resonant Converter With Wide Load Range.
In the practice in the art represented by these designs, the switching mode power supply is a DC power supply which regulates the average current applied to loads by changing the interrupting period (ON/OFF time within a period) by means of the switching controller. However, since the switching frequency of the switching mode power supply can not be made faster without limit, there is a limit as to designing the power supply device so as to be as small as possible. That is, the switching mode power supply can not be implemented as a power supply device of higher efficiency. Therefore, it is a problem that the power supply device becomes larger in size, in order to produce the desired power level. Therefore, if the switching mode power supply can generate the switching frequency at a high frequency, then the same can be implemented as a high-efficiency SMPS with a small size. In order to eliminate such problems, a power switching device of a resonant converter type generating a high switching frequency has been proposed. Such a power switching device of a resonant converter type adopts a partial resonance mode applied to the entire power supply device in which two switching elements must be operated at specified time intervals in order that such a partial resonance circuit can be normally operated.
In a resonant type power switching device, a start-up circuit produces trigger signals which are supplied to a pulse width modulation (PWM) controller at the time of initial start-up. The PWM controller produces a PWM signal for generating power in response to the trigger signal transferred from the start-up circuit at the time of initial start-up and after start-up produces PWM signals depending upon the switching of a phototransistor of a photocoupler. Such a PWM signal is a power control signal for generating switching power. A time delay delays the PWM signal prior to further transmitting it. A transistor forms a main switch which is connected between a primary transformer winding and a ground terminal and a gate electrode of the transistor is connected to an output terminal of the time delay. The main switch transistor is switched by the delayed PWM signal from the time delay, thereby generating switching power. A capacitor is connected between the primary transformer winding and the ground terminal in parallel to the main switch transistor.
The primary transformer winding has one end connected to the input terminal and has another end connected to the ground terminal via a diode and capacitor. The cathode electrode of the diode is connected to a supplementary transformer winding. Another transistor is connected between the input terminal and a capacitor serially connected to a resistor and further connected to a primary transformer winding. A gate electrode of the other transistor is connected to the input terminal of the PWM controller. An auxiliary switch transistor is connected between the supplementary transformer winding and the input terminal and a gate electrode of the supplementary transistor is connected to an output terminal of the other transistor. The auxiliary switch transistor is switched in response to the output of the other transistor to generate supplementary power.
Diodes and a capacitor, both connected to the secondary transformer winding, rectify and smooth the switching power induced in the secondary transformer winding. Additional resistors and a programmable reference diode compare the output power with a reference voltage and a photocoupler generates a switching pulse by means of the programmable reference diode.
The PWM controller produces a PWM signal for generating switching power and is applied to the other transistor and at the same time to the time delay thereby being delayed by a preset time prior to being further supplied to the main switching transistor. Then, the auxiliary switching transistor produces a switching signal in response to the switching of the other transistor.
The PWM controller initiates the power generating function of the entire device and the PWM signal applied to the main switching transistor is delayed by means of the time delay so as to main a specified off time interval after the other transistor is turned off in order to effect resonance operation. That is, in order to effect a partial resonance operation, the power switching device comprises the main switching transistor and the auxiliary switching transistor whereby an off time interval is arranged so as not to simultaneously turn on both of these transistors when driving them. For such a purpose, earlier power switching devices set a time delay when driving the main switching transistor.
These transistors are field effect transistors however, and in order to drive these switches, a high power drive circuit is needed. It is therefore very difficult to satisfy such high power driving and at the same time delay gate pulses by a necessary time interval.
I have discovered that the art fails to provide a power switching device capable of generating an off time interval by transferring a power control signal to the auxiliary switch in advance by a preset time interval.